mid-14c., "direct, undeviating, not crooked," properly "that which is stretched," adjectival use of Old English streht (altered, by analogy with streccan, from earlier streaht), past participle of streccan "to stretch" (see stretch (v.)). Meaning "true, direct, honest" is from 1520s. Of communication, "clear, unambiguous," from 1862. Sense of "undiluted, uncompromising" (e.g. straight whiskey, 1874) is American English, first recorded 1856.

Theatrical sense of "serious" (as opposed to popular or comic) is attested from 1895; vaudeville slang straight man first attested 1923. Go straight in the underworld slang sense is from 1919; straighten up "become respectable" is from 1907. Straight arrow "decent, conventional person" is 1969, from archetypal Native American brave name. To keep a straight face first recorded 1897; straight shooter is from 1928; straight-edge as a punk subculture is attested by 1987.

adj.2

"conventional," especially "heterosexual," 1941, probably in part from straight and narrow path "course of conventional morality and law-abiding behavior," which is based on a misreading of Matt. vii:14 (where the gate is actually strait), and the other influence seems to be from strait-laced.

n.

1864, "straight part of a race track," from straight (adj.1). Poker sense attested from 1841. Meaning "conventional person" is first recorded 1967 (see straight (adj.2)).